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September 26, 2013

Researchers Build World’s First Carbon Nanotube Computer

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redOrbit Staff & Stanford Reports - Your Universe Online

Stanford engineers have built the world’s first carbon nanotube computer, validating the concept of carbon nanotubes as a potential replacement for the conventional silicon chips used in modern electronic devices.

Carbon nanotubes (CNTs) are semiconductor material comprised of tens of thousands of tiny tube-like structures that can fit inside a human hair. The technology has the potential to launch a new generation of electronic devices that are smaller, run faster, and use less energy than those that rely on silicon chips.

In terms of performance, the CNT computer built by the Stanford researchers pales in comparison to modern computers, but the machine was still able to run a basic operating system and freely alternate between a program that counts in a loop and one that sorts numbers.

This first-of-its-kind accomplishment culminates years of efforts by scientists around the world to harness the potential of CNTs.

"People have been talking about a new era of carbon nanotube electronics moving beyond silicon," said lead researcher Subhasish Mitra, an electrical engineer and computer scientist and the Chambers Faculty Scholar of Engineering at Stanford.

"But there have been few demonstrations of complete digital systems using this exciting technology. Here is the proof."

Experts say the Stanford achievement will spur new efforts to find successors to silicon chips, which might soon encounter physical limits that prevent them from delivering smaller, faster and cheaper electronic devices.

"Carbon nanotubes (CNTs) have long been considered as a potential successor to the silicon transistor," said Professor Jan Rabaey, a world expert on electronic circuits and systems at University of California, Berkeley.

But until now it hasn't been clear that CNTs could fulfill those expectations.

"There is no question that this will get the attention of researchers in the semiconductor community and entice them to explore how this technology can lead to smaller, more energy-efficient processors in the next decade," Rabaey said.

CNTs were first fashioned into transistors – the on-off switches at the heart of digital electronic systems – about 15 years ago.

However, researchers working with the new technology faced frustrating challenges due to the imperfections in CNTs, slowing efforts to build complex circuits.

Professor Giovanni De Micheli, director of the Institute of Electrical Engineering at École Polytechnique Fédérale de Lausanne in Switzerland, highlighted two key contributions the Stanford team has made to this worldwide effort.

"First, they put in place a process for fabricating CNT-based circuits," De Micheli said. "Second, they built a simple but effective circuit that shows that computation is doable using CNTs."

Mitra said the work confirms that CNT technology can serve as a possible successor to silicon chips.

"It's not just about the CNT computer. It's about a change in directions that shows you can build something real using nanotechnologies that move beyond silicon and its cousins."

"There is no question that this will get the attention of researchers in the semiconductor community and entice them to explore how this technology can lead to smaller, more energy-efficient processors in the next decade," Rabaey said.

The move to replace silicon chips arises from the demands designers place upon semiconductors and transistors.

For decades, progress in electronics has meant shrinking the size of each transistor to pack more transistors on a chip. But as transistors become tinier, they waste more power and generate more heat in an increasingly smaller space. We can see this, for instance, by the warmth emanating from the bottom of a laptop.

Many researchers believe that this power-wasting phenomenon could spell the end of Moore's Law, which states that the density of transistors doubles roughly every two years, leading to smaller, faster and lower-cost electronics.

However, smaller, faster and cheaper has also meant smaller, faster and hotter.

"Energy dissipation of silicon-based systems has been a major concern," said Anantha Chandrakasan, head of electrical engineering and computer science at MIT and a world leader in chip research.

Chandrakasan called the Stanford work "a major benchmark" in moving CNTs toward practical use.

CNTs are long chains of carbon atoms that are extremely efficient at conducting and controlling electricity. In fact, they are so thin that it takes very little energy to switch them off, said Stanford professor H.S. Philip Wong, co-author of a paper about the current work published in Nature.

"Think of it as stepping on a garden hose," Wong said. "The thinner the hose, the easier it is to shut off the flow."

In theory, this combination of efficient conductivity and low-power switching make CNTs excellent candidates to serve as electronic transistors.

"CNTs could take us at least an order of magnitude in performance beyond where you can project silicon could take us," Wong said.

But inherent imperfections have stood in the way of putting this promising material to practical use. To begin with, CNTs do not necessarily grow in neat parallel lines, as chipmakers would like. Over time, researchers have devised tricks to grow 99.5 percent of CNTs in straight lines. But with billions of nanotubes on a chip, even a tiny degree of misaligned tubes could cause errors, so the problem remained.

A second type of imperfection has also thwarted CNT technology. Depending on how the CNTs grow, a fraction of them can end up behaving like metallic wires that always conduct electricity, instead of acting like semiconductors that can be switched off.

Since mass commercial production is the ultimate goal, researchers had to find ways to manage misaligned and metallic CNTs without having to search for them like needles in a haystack.

"We needed a way to design circuits without having to look for imperfections or even know where they were," Mitra said.

The Stanford paper describes a two-pronged approach that the authors describe as an "imperfection-immune design."

To eliminate the wire-like or metallic nanotubes, the Stanford team switched off all the good CNTs, then pumped the semiconductor circuit full of electricity. All of that electricity concentrated in the metallic nanotubes, which grew so hot that they burned up and literally vaporized into tiny puffs of carbon dioxide. This sophisticated technique was able to eliminate virtually all of the metallic CNTs in the circuit at once.

Bypassing the misaligned nanotubes required even greater delicacy, so the researchers created a powerful algorithm that maps out a circuit layout that is guaranteed to work, regardless of whether or where CNTs might be askew.

The Stanford team used this imperfection-immune design to assemble a basic computer with 178 transistors, a limit imposed by the fact that they used the university's chip-making facilities rather than an industrial fabrication process. Each transistor was composed of 10 to 200 nanotubes, and the machine’s footprint was less than 7 square millimeters.

The CNT computer ran a basic operating system that allowed the machine to alternate between tasks. Like modern-day computers, the memory on the CNT machine was “off-chip” and not made up of nanotubes.

In a demonstration of its potential, the researchers also showed that their CNT computer could run MIPS, a commercial instruction set developed in the early 1980s by then Stanford engineering professor and now university President John Hennessy.

Although it could take years to mature, the Stanford approach validates the possibility of industrial-scale production of carbon nanotube semiconductors, said Naresh Shanbhag, a professor at the University of Illinois at Urbana-Champaign and director of SONIC, a consortium of next-generation chip design research.